Commercially available previously-known stents typically are either balloon expandable or self expanding. Both types of stents suffer from a host of drawbacks, including recoil, foreshortening and, in the case of self-expanding stents, imprecision in deployment location.
Balloon expandable stents are generally made from a material that is readily plastically deformed into two directions. Before insertion, the stent is placed around the balloon section at the distal end of a catheter and crimped onto the balloon. Once the stent is positioned at a target location within the body, it is deployed by inflating the balloon to plastically deform the stent. The stent is at its largest diameter and should function to support the surrounding tissue when deployed, and to prevent an undesired reversion to a smaller diameter.
Therefore, stents generally require sufficient rigidity in the radial direction to retain the vessel patent, but also some flexibility in the axial direction when deployed so as to not reform the vessel. Further, the stent preferably should use as small an amount of material as possible and its interior surface should not obstruct the flow through the channel (e.g., for blood) or cause too much turbulence.
A number of problems are generally known to occur with balloon expandable stents. For example, after the stent is compressed to its smallest diameter around the balloon of the delivery catheter, the stent is subject to some elastic return to a slightly larger diameter. This may cause problems when the catheter is brought into the patient's body, for example, if friction between balloon and stent becomes so small that the stent slips off the catheter. Further, the larger size of the stent disadvantageously presents a larger delivery profile.
A further problem with previously-known commercially available balloon expandable stents is so-called “recoil.” Recoil refers to the fact that, after expansion by the balloon pressure, the outer diameter of the stent becomes slightly smaller when the balloon is deflated. This effect may result in the stent having as much as a 10% decrease in deployed diameter, which can lead to undesirable migration of the stent. In addition, in an effort to offset recoil, a clinician may overexpand the stent during balloon inflation, which may lead to excessive trauma to the vessel endothelium and exacerbate restenosis.
Self-expanding stents are made of a more or less elastically expanding structure, which has to be held on the catheter by some external means. An example of this type is a stent that is held in its constrained state by a delivery sheath. The stent deploys to its expanded shape when the sheath is removed. Self-expanding stents also may be constructed of a shape memory material that exhibits either superelastic behavior or which is expanded by a temperature change. Self-expanding stents suffer from a number of drawbacks, including increased delivery profile resulting from use of a delivery sheath and inaccurate placement resulting from stent movement during deployment.
The foregoing previously-known types of stents further have the disadvantage of relatively large length change during expansion and a poor hydrodynamic behavior because of the shape of the metal wires or struts.
Another disadvantage of some stents is the positive spring rate, which means that further expansion can only be achieved by higher balloon pressure.
The construction of prior stents is typically made in such a way that the external forces, working on the stent in the radial direction, merely cause bending forces on the struts or wires of the structure.
For example, a unit cell of a Palmaz-Schatz-stent, as produced by Cordis Corporation has in its collapsed condition a flat, rectangular shape and in its expanded condition a more or less diamond-shaped form with almost straight struts or curved struts.
The shape of the unit cell of such stents is typically symmetrical with four struts each having the same cross section. In addition, the loading of the cell in the axial direction typically will cause an elastic or plastic deformation of all of the struts, resulting in an elongation of the unit cell in the radial direction. These unit cells have a positive spring rate. In stents having such unit cells, the stability against radial pressure is dependent primarily on the banding strength of the struts and their connections.
To address some of the concerns encountered with previously-known stents, especially restenosis, it has been suggested to make stents out of a biodegradable material. One example of such a stent is described in U.S. Pat. No. 5,449,382 to Dayton, which describes rolled-sheet stents made, for example, of biodegradable polylactic acid polymers or polyglycolic acid polymers. The stents are impregnated with a drug that is released into the vessel during biodegradation of the stent to reduce thrombosis or restenosis.
U.S. Pat. No. 6,488,702 to Besselink, of which the present application is a continuation-in-part, discloses a new type of unit cell that may be incorporated into a stent to address the shortcomings of previously-known stents. That patent discloses a tubular device, such as a stent, comprising a plurality of bistable or multistable unit cells. Those unit cells are described as having only a discrete number of stable configurations, and snap from one stable configuration to the next upon application of a suitably directed radial force. The devices described in that patent advantageously overcome many of the problems encountered with previously-known plastically deformable and self-expanding stents.
The aforementioned Besselink patent describes that devices having bistable and multistable unit cell structures may be constructed of metal alloys, polymers and shape memory materials, such as nickel-titanium alloys. When configured as stents, such structures will endothelialize once deployed within a body vessel. However, situations may arise where it would be desirable to capture the benefits afforded by using a bistable or multistable unit cell in conjunction with a drug eluting capability.
It would therefore be desirable to provide a stent comprised of bistable or multistable unit cells, and which is formed from a biodegradable material.
It further would be desirable to provide a stent comprised of bistable or multistable unit cells, and which provides in addition a drug delivery capability.